Bentonite is supplied in granular form to initially allow for water to penetrate and come into contact with the particles. As water is absorbed, the clay expands to fill cavities both between the granules and adjacent soil. Once hydrated, the clay forms an impermeable barrier.

Description and Use:

Bulk sodium bentonite best suited to major contracts requiring mechanical handling and placement.

Sodium bentonite expands when wet making it an industry standard as a low permeability, self sealing natural geo-technical barrier,

Natural "as dug" Sodium bentonite is the same product that is extensively used for oil and geothermal drilling muds. Here the ratios of clay to water (and other additives) are closely monitored, however, for the sealing of ponds, streams, and generally preventing the passage of ground water the application is more an art form than an exact science.

The product can be introduced direct into the ground, allowing for natural moisture to be taken up by the clay. For example low permeability barriers can be achieved by cutting a slit trench and simply pouring the clay into the excavation. ( or, "sandwich" the clay between geosynthetic (weed) membrane to provide a highly effective impermeable barrier) Moisture in the ground will cause the clay particles to swell and form a seal. This can be particularly effective to constrain root growth around newly planted trees, the bentonite barrier preventing damage to foundations and service ducts.

For natural ponds a detailed explanation of using the dry clay granules to seal a pre-formed hollow can be found on the internet at www.youtube.com/watch?v=7ltqNH7eoEM. The process involves mixing the granules into the sub-soil and then compacting the mix. The video explains the need to keep the sides of the pond relatively shallow and (the part 2 video) clearly demonstrates the best way to fill the completed structure so as not to disturb the seal.

This method, or a variation where the clay is dry mixed with excavated soils in a cement mixer, re-introduced into the pond shape and then compacted with a wacker plate (or hob nail boots like Georgian canal builders !).

Unfortunately no absolutes can be given with regard to the proportion of granules to sub-soil for this method. This needs to be established on site with the actual soils being excavated. However, the proportion Approx 10kg per Sqm ( 2lbs per sqft) as described in the above video provides a good starting guide for a “sandy clay” sub soil.

Sodium bentonite is used in electrical and lightening earth pit installations as a non corrosive linkage into natural soils. The clay holds moisture within its mineralogical structure and thus mitigates against seasonal variation in ground resistance levels.

Occurrence:
Bentonite is a variety of clay which is predominantly composed of the mineral Montmorillonite. The clays are thought to be a product derived from the decomposition of volcanic ash, and are worked from beds of Cretaceous and Tertiary age in the Western United States.